The image forming apparatus of an electrophotographic type such as copying machines or laser printers forms a toner image according to image information on a photoconductive drum, transfers the same on a transfer medium, and finally forms an image on a sheet of paper as a printing object. For example, a laser beam printer forms an electrostatic latent image by charging the surface of the photoconductive drum to a predetermined background potential, and then exposing the surface of the photoconductive drum with a laser beam modulated by the image information. The electrostatic latent image is developed by toner to form a toner image, and the toner image is transferred to a transfer medium.
There are two such image forming methods as shown below. In other words, there are a method of transferring a toner image directly from the photoconductive drum to a sheet of paper as a printing object, which is a final transfer medium, and a method of primarily transferring a toner image on an intermediate transfer belt of an endless film shape, which is used as an intermediate transfer medium, from the photoconductive drum and then secondarily transferring the same to a sheet of paper as the printing object, which is the final transfer medium.
The method of transferring using the intermediate transfer belt from these two methods is employed for forming a full color image, which requires overlapping of toner images in plurality of colors.
In both the transfer methods described above, a transfer roller for transferring the toner image to the intermediate transfer belt or the sheet of paper as the printing object (hereinafter, referred generally to as transfer medium). The transfer roller is arranged at a position opposing the photoconductive drum with the intermediary of the transfer medium, and electrostatically transfers the toner image to the surface of the transfer medium by providing an electric charge having an opposite polarity from the polarity of an electric charge of the toner to the back surface of the transfer medium. In other words, when the polarity of the charge of the toner is minus (−), a transfer bias of plus (+) is applied to the transfer roller to achieve the electrostatic transfer.
Factors which affect on the electrostatic transfer of the toner image as described above include environmental factors such as the temperature or the humidity. In general, a value of resistance of the transfer roller increases under a low-temperature and low-moisture environment, and the value of resistance of the transfer roller is lowered under a high-temperature and high-moisture environment. It is the same for the value of resistance of the transfer medium. When these environmental factors vary, a sufficient transfer electric field cannot be generated between the toner image and the transfer medium even though the same transfer bias is applied, so that deterioration of the transfer efficiency is resulted.
In order to solve the problem as described above, the apparatus disclosed in Japanese Unexamined Patent Application Publication No. JP-A-2000-116641 uses an environmental sensor, and employs a control system to measure the values of resistance of a conductive roller in each environmental condition and achieve an optimal voltage setting at a printing unit by controlling the applied voltage on the basis of a measured value of resistance. However, such control requires a control device having various sensors and storage devices, which results in complication of the apparatus and cost increase.
With means for measuring the resistance of the conductive roller employing a control system to output a predetermined current in a resistance sensing process, calculate the resistance from a measured value of applied voltage and achieve the optimal voltage setting at the printing unit, avoidance of the complication of the apparatus and the cost increase described above is possible. However, there is a case where a stabilized transfer efficiency cannot be obtained in various environments from the reasons shown below.
In the case of a color transfer, since the toners in a plurality of colors are transferred in an overlapped manner as described above, the percentage occupied by a value of toner resistance is increased in an environment in which the value of resistance of the conductive roller remarkably drops. Therefore, the optimal voltage setting varies depending on the thickness of a transferred toner layer, and hence the transfer efficiency may vary depending on the attached amount of toner.
In addition, when forming a full color image, it is necessary to consider a problem of reverse transfer due to a white background inrush current. In other words, when forming the full color image, the toner images are transferred from photoconductive members to the intermediate transfer belt at respective transfer positions for yellow, magenta, cyan, and black in sequence. However, toner in a specific color is not used depending on the image. For example, when black toner is not used, and hence the black toner is not present on the photoconductive drum at a minus potential, if a plus potential is applied from the transfer roller at the transfer position for black, a significant inrush current flows because there is no toner, which is a high resistance material. When such an inrush current flows, toner which is transferred to the intermediate transfer belt already in the upstream transfer devices for yellow, magenta, and cyan and is already charged in minus is inverted into a straight polarity, so that an inverted transfer phenomenon in which the toner is adhered to the photoconductive drum from the intermediate transfer belt occurs. When occurrence of such a reverse transfer is prominence, a defective color balance may occur in an output image. Therefore, a countermeasure for the white background inrush current is necessary.